Cured phenolic resins are generally prepared by compounding formaldehyde, or a substance capable of releasing formaldehyde such as hexamethylene-tetramine, with a fusible condensation product of phenol and formaldehyde, and curing the mixture. This is known as the two-stage process and the fusible condensation product is known as a novolak. Novolaks are intentionally prepared with a deficiency of formaldehyde, and will not cure by themselves. In order to cure the novolak, some form of aldehyde or aldehyde donor must be added to bring the ratio of aldehyde to phenol to the proper level for curing. Commercially, hexamethylenetetramine is commonly used and is blended in finely divided form with the pulverized resin. Under the influence of heat, the hexamethylenetetramine breaks down into formaldehyde and ammonia, and the formaldehyde reacts with the novolak under alkaline conditions. The end result is the formation of a network by the cross-linking of the many linear and branched polymer chains formed during previous reactions.
In the production of molded articles, the phenolic molding compound is conventionally mixed in a barrel prior to injection molding. The barrel is maintained at a temperature that is somewhat less than the molding temperature. The barrel temperature should be high enough so that the resin is sufficiently fluid to be injection molded, and yet low enough so that the resin will not prematurely cure or harden. A barrel temperature that is too low will cause the resin to become too viscous, and ultimately granular or powdery. If the barrel temperature is maintained at a level that is too high, the resin will harden prior to injection molding.
As is known to those skilled in the art, hexamethylene-tetramine may decompose and form formaldehyde and ammonia at temperatures of as low as 90.degree. C when contacted with a phenolic resin, causing the resin to cross-link prematurely. Since the rate of cross-linking increases with increasing temperature, the barrel life at the barrel temperature (usually about 125.degree. C) is shorter than would be commercially desirable, after which the phenolic molding compound loses fluidity and cannot be injected. Consequently, it would be advantageous to increase the time available for the molding process by extending the barrel life of the resin.
Accordingly, it is an object of this invention to provide a cross-linking or curing agent for phenolic molding compounds which is thermally stable below the molding temperature, but which will be sufficiently reactive at the molding temperature so that the rate of curing of the resin, or the molding cycle, will not be significantly affected. It is a further object to provide a curing agent which will extend the barrel life of phenolic molding compounds and increase the efficiency of the molding operation.